The subjects will be patients with MAS and non-suppressible growth hormone as determined by standard oral glucose tolerant testing (OGTT) and an elevated insulin-like growth factor-1 (IGF-I). It will be a randomized, blinded crossover design. The primary and secondary measures of efficacy will be: the normalization of serum (IGF-I), a reduction in signs and symptoms of growth hormone excess, and a net change in Insulin-like Growth Factor Binding Protein 3 (IGFBP-3). The effect of pegvisomant on the fibrous dysplastic bone activity in these patients will be determined by a net change in the levels of bone turnover markers.